What is the difference between "px", "dip", "dp" and "sp"?

From the Android Developer Documentation:

1. px
   > Pixels - corresponds to actual pixels on the screen.

2. in
   > Inches - based on the physical size of the screen.
   > 1 Inch = 2.54 centimeters

3. mm
   > Millimeters - based on the physical size of the screen.

4. pt
   > Points - 1/72 of an inch based on the physical size of the screen.

5. dp or dip
   > Density-independent Pixels - an abstract unit that is based on the physical density of the screen. These units are relative to a 160dpi screen, so one dp is one pixel on a 160 dpi screen. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion. Note: The compiler accepts both "dip" and "dp", though "dp" is more consistent with "sp".

6. sp
   > Scaleable Pixels OR scale-independent pixels - this is like the dp unit, but it is also scaled by the user's font size preference. It is recommended youuse this unit when specifying font sizes, so they will be adjustedfor both the screen density and the user's preference. Note, the Android documentation is inconsistent on what sp actually stands for, one doc says "scale-independent pixels", the other says "scaleable pixels".

From Understanding Density Independence In Android:

More info can be also be found in the Google Design Documentation.

Pretty much everything about this and how to achieve the best support for multiple screens of different sizes and densities is very well documented here:

• Supporting Multiple Screens

Screen size
Actual physical size, measured as the screen's diagonal.For simplicity, Android groups all actual screen sizes into fourgeneralized sizes: small, normal, large, and extra-large.

Screen density
The number of pixels within a physical area of thescreen; usually referred to as dpi (dots per inch). For example, a"low" density screen has fewer pixels within a given physical area,compared to a "normal" or "high" density screen. For simplicity,Android groups all actual screen densities into six generalizeddensities: low, medium, high, extra-high, extra-extra-high, andextra-extra-extra-high.

Orientation
The orientation of the screen from the user's point ofview. This is either landscape or portrait, meaning that the screen'saspect ratio is either wide or tall, respectively. Be aware that notonly do different devices operate in different orientations bydefault, but the orientation can change at runtime when the userrotates the device.

Resolution
The total number of physical pixels ona screen. When adding support for multiple screens, applications donot work directly with resolution; applications should be concernedonly with screen size and density, as specified by the generalizedsize and density groups.

Density-independent pixel (dp)
A virtualpixel unit that you should use when defining UI layout, to expresslayout dimensions or position in a density-independent way.The density-independent pixel is equivalent to one physical pixel on a 160dpi screen, which is the baseline density assumed by the system for a"medium" density screen. At runtime, the system transparently handlesany scaling of the dp units, as necessary, based on the actual densityof the screen in use. The conversion of dp units to screen pixels issimple:px = dp * (dpi / 160).For example, on a 240 dpi screen, 1 dpequals 1.5 physical pixels. You should always use dp units whendefining your application's UI, to ensure proper display of your UI onscreens with different densities.

If you are at all serious about developing an Android app for more than one type of device, you should have read the screens support development document at least once. In addition to that, it is always a good thing to know the actual number of active devices that have a particular screen configuration.

• Screen Sizes and Densities

I will elaborate more on how exactly does dp convert to px:

• If running on an mdpi device, a 150 x 150 px image will take up 150 * 150 dp of screen space.
• If running on an hdpi device, a 150 x 150 px image will take up 100 * 100 dp of screen space.
• If running on an xhdpi device, a 150x150 px image will take up 75 * 75 dp of screen space.

The other way around: say, you want to add an image to your application and you need it to fill a 100 * 100 dp control. You'll need to create different size images for supported screen sizes:

• 100 * 100 px image for mdpi
• 150 * 150 px image for hdpi
• 200 * 200 px image for xhdpi